The present invention relates to a thin film semiconductor device comprising a thin film transistor formed on an insulation substrate and having a polycrystalline semiconductor layer to define an active region. More specifically, the present invention relates to the technique of hydrogenation treatment of thin film semiconductors.
Referring to FIG. 17, a process for hydrogenation treatment is described briefly below. As shown in the figure, a thin film of polycrystalline silicon 102 patterned to a predetermined shape is formed on the surface of an insulation substrate 101 to provide an element region. A source region S and a drain region D containing impurities at high concentration are formed in the thin film of polycrystalline silicon 102, and a channel region Ch is incorporated therebetween. A thin film transistor (TFT) is established by forming a gate electrode G on the upper side of the channel region Ch, with a gate oxide film 103 and a gate nitride film 104 interposed therebetween. The resulting TFT is coated with a first interlayer insulating layer 105, and an interconnecting electrode 106 is connected to the source region S via a first contact hole provided in the first interlayer dielectric film. A second interlayer insulating layer 107 is further deposited on the first insulation film 105. A pixel electrode 108 made of a light-transmitting electrically conductive film such as an ITO (indium tin oxide) film is formed on the second interlayer insulating layer 107 by patterning, and is electrically connected with the drain region D of the TFT via a second contact hole. A P-SiN film 109 is formed as an overpassivation film on the surface of the second interlayer insulating layer film 107 by patterning. The P-SiN film 109 is a relatively porous film, and contains hydrogen atoms at a considerable quantity. Accordingly, the P-SiN film serves as a hydrogen supply source. Thus, by depositing a P-SiN film 109 after forming a TFT and annealing it thereafter, the hydrogen atoms are allowed to diffuse into the thin film of polycrystalline silicon 102 through the second interlayer insulating layer film 107, the first interlayer dielectric film 105, the gate oxide film 103, and the like. The hydrogen atoms introduced by the hydrogenation treatment diffuse into the grain boundaries of the thin film of polycrystalline silicon 102 to finally combine with the dangling bonds. Accordingly, the barrier potential is lowered due to the decrease in the density of traps. The carrier mobility inside the polycrystalline silicon TFT is thereby elevated to increase the ON current. The leak current can be suppressed by the decrease in trap levels. Furthermore, the threshold voltage of the transistor can be lowered because a part of the hydrogen atoms diffuses into the grain boundary of the thin film of polycrystalline silicon 102 to combine with the dangling bonds, thereby lowering the barrier potential by decreasing the trap density. Thus, the carrier mobility inside the polycrystalline silicon TFT can be further elevated to increase the ON current. Moreover, the leak current can be controlled by reducing the trap levels. Furthermore, a part of the hydrogen atoms introduced by the treatment also combine with the boundary levels at the boundary between the polycrystalline silicon layer and the gate oxide film to further lower the threshold voltage of the transistor.
In the technique described in the foregoing, the P-SiN film 109 provided as the diffusion source contains hydrogen to a considerable quantity. Accordingly, it may possibly undergo reduction reaction with ITO which constitutes the pixel electrode 108. This requires the part of the P-SiN film in contact with ITO to be removed by photolithography and etching to prevent the reduction reaction from occurring between the P-SiN film and ITO. However, these additional steps for removal consume both time and cost. Moreover, it is difficult to realize TFTs of uniform quality once a part of the P-SiN film is removed, because the hydrogenation efficiency decreases in the part from which the P-SiN is removed. As an alternative means for hydrogenation, a TFT is exposed to hydrogen plasma to incorporate hydrogen into the structure. Similar to the process using a P-SiN film as the hydrogen supply source, this process also requires additional steps, special apparatuses, etc., thereby leading to an increased cost and process time.